<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <!--
        viewport  移动端的属性，在 pc 没有任何作用。

    -->
    <meta name="viewport" content="width=device-width,user-scalable=no,initial-scale=1.0">
    <style>
            body{
                margin: 0;
                padding: 0;
            }
            .layout{
                    min-width: 320px;
                    max-width: 640px;
                    height: 100px;
                    background: red;
                    margin: 0 auto;
            }
    </style>
</head>
<body>
    <div class="layout"></div>
</body>
</html>